1. Field of the Invention
The present invention relates to a piezoelectric single crystal and a fabrication method therefor. More particularly, the invention relates to a piezoelectric single crystal which is developed paying attention to an electromechanical coupling factor in a direction perpendicular to the polarization direction, i.e., a lateral vibration mode, and domain control in that direction, and a fabrication method therefor.
2. Description of the Related Art
With regard to a piezoelectric single crystal, for example, Japanese Patent Laid-Open No. 38963/1994 discloses an ultrasonic probe using a piezoelectric material comprised of a solid solution single crystal of lead zinc niobate-lead titanate. This technique provides a probe with a high sensitivity by using that single crystal of such a piezoelectric material which has an electromechanical coupling factor (k33) of 80 to 85% in a direction parallel to the polarization direction. While the electromechanical coupling factors in the direction parallel to the polarization direction of piezoelectric single crystals have been studied and various usages have been developed conventionally, the characteristics in a direction perpendicular to the polarization direction have not been studied yet.
The present inventors paid attention to the facts that a piezoelectric single crystal is adapted to multifarious usages for the electromechanical coupling factor (k33) in a direction parallel to the polarization direction (longitudinal vibration mode) of the piezoelectric single crystal has a value equal to or greater than 80%, the electromechanical coupling factor (k31) in a direction perpendicular to the polarization direction (lateral (=length extensional) vibration mode) is, for example, 49% to 62%, lower than the electromechanical coupling factor k33 in the direction parallel to the polarization direction (longitudinal vibration mode), as described in pp. 239 in IEEE Proc. MEDICAL IMAGING 3664 (1999) and other documents, and that the electromechanical coupling factor k31 takes a value that varies from one document to another. Through intensive studies on the phenomenon, the inventors discovered that it would be possible to fabricate a piezoelectric single crystal and their devices which would effectively use the electromechanical coupling factor k31 in case where the electromechanical coupling factor k33 in the direction parallel to the polarization direction (longitudinal vibration mode) was equal to or greater than 80%, a piezoelectric constant d33 was equal to or greater than 800 pC/N, the electromechanical coupling factor k31 was equal to or greater than 70% and a piezoelectric constant xe2x88x92d31 was equal to or greater than 1200 pC/N (d31 having a negative value by definition), and would be possible to fabricate a piezoelectric single crystal and their devices which would use the value of k33 more efficiently due to generation of no spurious (undesired vibration) or the like in the band of usage of that value in case where k33 was equal to or greater than 80%, d33 was equal to or greater than 800 pC/N, k31 was equal to or smaller than 30% and xe2x88x92d31 was equal to or smaller than 300 pC/N (d31 having a negative value by definition).
Further, the inventors discovered that the cause for the piezoelectric single crystal to have an intermediate electromechanical coupling factor k31 in the direction perpendicular to the polarization direction (lateral vibration mode) and to have a variation in the factor while having a large electromechanical coupling factor k33 in the direction parallel to the polarization direction (longitudinal vibration mode) was that the domain structure formed by an electric dipole associated with the direction perpendicular to the polarization direction of a polarized piezoelectric single crystal was formed by plural domains (multiple domains), not a single domain, and that the following piezoelectric single crystals (A) and (B) were obtained by controlling the domain structure.
(A) A domain controlled piezoelectric single crystal having an electromechanical coupling factor k33xe2x89xa780% in the longitudinal vibration mode and a piezoelectric constant d33xe2x89xa7800 pC/N, comprising an electromechanical coupling factor k31xe2x89xa770% in a lateral vibration mode, a piezoelectric constant xe2x88x92d31xe2x89xa71200 pC/N and a frequency constant fc31 (=frxc2x7L)xe2x89xa6650 Hzxc2x7m which is a product of a resonance frequency fr in the lateral vibration mode relating to k31 and a length L of the piezoelectric single crystal in a vibration direction.
(B) A domain controlled piezoelectric single crystal having an electromechanical coupling factor k3324 80% in a longitudinal vibration mode and a piezoelectric constant d33xe2x89xa7800 pC/N, comprising an electromechanical coupling factor k31xe2x89xa630% in a lateral vibration mode in a direction perpendicular to the polarization direction, a piezoelectric constant xe2x88x92d31xe2x89xa6300 pC/N and a frequency constant fc31 (=frxc2x7L)xe2x89xa7800 Hzxc2x7m which is a product of a resonance frequency fr in the lateral vibration mode relating to k31 and a length L of the piezoelectric single crystal in a vibration direction.
The inventors also found that the conditions for controlling the domain structure were rearranged based on the value of a frequency constant fc31 (=frxc2x7L), which is a product of the resonance frequency fr in the lateral vibration mode relating to k31 and the length L of the piezoelectric single crystal in the vibration direction.
The invention aims at providing such a piezoelectric single crystal and a fabrication method therefor.
According to the first aspect of the invention, there is provided a domain controlled piezoelectric, single crystal having an electromechanical coupling factor k33xe2x89xa780% in a longitudinal vibration mode and a piezoelectric constant d33xe2x89xa7800 pC/N, comprising an electromechanical coupling factor k31xe2x89xa770% in a lateral vibration mode, a piezoelectric constant xe2x88x92d31xe2x89xa71200 pC/N (d31 has a negative value by definition) and a frequency constant fc31 (=frxc2x7L)xe2x89xa6650 Hzxc2x7m which is a product of a resonance frequency fr in the lateral vibration mode relating to k31 and a length L of the piezoelectric single crystal in a vibration direction.
According to the second aspect of the invention, there is provided a domain controlled piezoelectric single crystal having an electromechanical coupling factor k33xe2x89xa780% in a longitudinal vibration mode and a piezoelectric constant d33xe2x89xa7800 pC/N, comprising an electromechanical coupling factor k31xe2x89xa630% in a lateral vibration mode, a piezoelectric constant xe2x88x92d3xe2x89xa6300 pC/N (d31 has a negative value by definition) and a frequency constant fc31 (=frxc2x7L) xe2x89xa7800 Hzxc2x7m which is a product of a resonance frequency fr in the lateral vibration mode relating to k31 and a length L of the piezoelectric single crystal in a vibration direction.
The lengthwise direction of, for example, a rod-like piezoelectric single crystal with an aspect ratio of 3 or greater is the polarization direction and the vibration in the direction parallel to the polarization direction (longitudinal vibration) when a voltage is applied in the polarization direction and the efficiency of conversion between electrical and mechanical energy are respectively expressed by the electromechanical coupling factor k33 in longitudinal vibration mode and the piezoelectric constant d33. The greater those values are, the higher the efficiency is. Those values are also defined for piezoelectric single crystals with other shapes, such as a plate and a disc shape, besides the rod-like one. The invention pertains to a domain controlled piezoelectric single crystal developed paying attention to the electromechanical coupling factor k31 in the direction perpendicular to the polarization direction (lateral vibration mode).
It is possible to use the following materials as the piezoelectric single crystal material according to the first aspect of the invention or the second aspect of the invention.
(a) A solid solution which is comprised of Xxc2x7Pb (A1, A2, . . . , B1, B2, . . . )O3+(1xe2x88x92X)PbTiO3(0 less than X less than 1) where A1, A2, . . . are one or plural elements selected from a group of Zn, Mg, Ni, Lu, In and Sc and B1, B2, . . . are one or plural elements selected from a group of Nb, Ta, Mo and W, and in which a sum xcfx89 of valencies of elements in parentheses in a chemical formula Pb(A1Y1 a1, A2Y2 a2, . . . , B1Z1 b1, B1Z2 b2, . . . )O3 satisfies charges of xcfx89=a1xc2x7Y1+a2xc2x7Y2+. . . b1xc2x7Z1+b2xc2x7Z2+. . . =4+ where a1, a2, . . . are valencies of A1, A2, . . . , b1, b2, . . . are valencies of B1, B2, . . . and Z1, Z2, . . . are composition ratios in the chemical formula.
(b) The material (a) added with 0.5 ppm to 1% by mass of one or two of Mn and Cr.
The best known materials are piezoelectric single crystal materials comprised of solid solutions of lead zinc niobate Pb(Zn1/3Nb2/3)O3 or lead magnesium niobate Pb(Mg1/3Nb2/3)O3 and lead titanate PbTiO3 (the former combination is called xe2x80x9cPZN-PTxe2x80x9d or xe2x80x9cPZNTxe2x80x9d and the latter one xe2x80x9cPMN-PTxe2x80x9d or xe2x80x9cPMNTxe2x80x9d).
The following are methods of fabricating the above-described domain controlled piezoelectric single crystals.
The first one is a piezoelectric single crystal fabricating method of fabricating a domain controlled piezoelectric single crystal, comprising a step of applying a DC electric field of 400 V/mm to 1500 V/mm for a maximum of two hours in a temperature range of 20xc2x0 C. to 200xc2x0 C. as polarization conditions in a thickness direction of the piezoelectric single crystal or a step of cooling while applying an electric field (field applied cooling).
While this method performs final polarization of a domain controlled piezoelectric single crystal device such as actuators, sensors and transducers, it is effective to employ a piezoelectric single crystal device such as actuators, sensors and transducers fabricating method which, prior to the final polarization step, additionally comprises a step of applying an electric field in a direction perpendicular to the polarization direction of a piezoelectric single crystal and a step of controlling a direction of a ferroelectric domain perpendicular to the polarization direction. The types of electric fields to be applied in the direction perpendicular to the polarization direction include steady electric fields, such as a DC electric field, a pulse electric field and an AC electric field, and an attenuating electric field. There are different proper conditions for the intensities of the electric fields, the application times, temperatures and so forth depending on the characteristics of each piezoelectric single crystal and the desired value of the electromechanical coupling factor k31 in the direction perpendicular to the polarization direction. Those conditions can be determined by experiments, etc. As the pulse electric field, a unipolar pulse, such as a rectangular wave, and a bipolar pulse, such as an (AC) triangular wave can be used.
Another method of the invention is characterized by heating and cooling a piezoelectric single crystal before a step of final polarization of a domain controlled piezoelectric single crystal, which applies a DC electric field of 400 V/mm to 1500 V/mm for a maximum of two hours in a temperature range of 20xc2x0 C. to 200xc2x0 C. or performs cooling while applying an electric field (field applied cooling). For example, the temperature ranges for a piezoelectric single crystal to become rhombohedral, tetragonal and cubic are determined in accordance with the composition. Therefore, as a step (1) of heating and cooling a piezoelectric single crystal material with a temperature of transition between a rhombohedral crystal which is in a low temperature phase of the piezoelectric single crystal and a tetragonal crystal which is in an intermediate temperature phase of the piezoelectric single crystal material in between, or a step (2) of heating and cooling the piezoelectric single crystal material between a Curie temperature Tc (the piezoelectric single crystal material becomes cubic over the Tc with the disappearance of the ferroelectricity) and a rhombohedral crystal in the low temperature phase or a tetragonal crystal in the intermediate temperature phase, or a step (3) of heating and cooling the piezoelectric single crystal material in a temperature range of a cubic crystal which is in a high temperature phase equal to or higher than the Tc, or a step (4) of adequately combining the steps (1),(2) and (3) is executed followed by a step of applying a DC electric field of 400 V/mm to 1500 V/mm for a maximum of two hours in a temperature range of 20xc2x0 C. to 200xc2x0 C. or performing cooling while applying an electric field, it is possible to control the aligned state of ferroelectric domains perpendicular to the polarization direction.
Further, before a step of applying a DC electric field of 400 V/mm to 1500 V/mm for a maximum of two hours in a temperature range of 20xc2x0 C. to 200xc2x0 C. or performing cooling while applying an electric field, a step of applying an electric field in a direction perpendicular to the polarization direction of a piezoelectric single crystal and a step of heating and cooling the piezoelectric single crystal are used together, followed by a step of applying a DC electric field of 400 V/mm to 1500 V/mm for a maximum of two hours in a temperature range of 20xc2x0 C. to 200xc2x0 C. or performing cooling while applying an electric field, it is possible to control the aligned state of ferroelectric domains perpendicular to the polarization direction.